Generally, milling machining refers to a process of machining a workpiece made of steel or cast iron into a desired shape by means of a rotating cutting tool or cutter with cutting inserts mounted thereon. Milling machining includes flank machining, right angle machining, drilling, grooving, etc. Among these, right angle machining refers to a milling process of forming a corner portion where two surfaces meet each other at an angle of 90°. Since right angle machining machines the two surfaces at a time, the length of a cutting edge participating in cutting is relatively long, and thus, the entire cutting edge is subjected to more resistance. Accordingly, right angle machining needs to effectively disperse such resistance and enhance the service life of a tool.
As an example of a conventional cutting insert for right angle machining, there is known in the art a single-sided cutting insert that has a cutting edge only between an upper surface and a flank surface and has no cutting edge between a lower surface and the flank surface. Such a single-sided cutting insert, however, has a short life expectancy and is inefficient due to few available cutting edges.
Further, there is known in the art a cutting insert for right angle machining, which has cutting edges between a lower surface and a flank surface as well as between an upper surface and the flank surface and thus can be used with its double sides. By way of example, Korean Patent Application Publication No. 10-2008-0041260 discloses a cutting insert wherein an upper surface and a lower surface meet a flank surface at an angle of 90°. According to the abovementioned reference, to ensure a relief angle when machining a workpiece, a cutting insert must be positioned in such a manner that an angle formed by a cutting edge of the cutting insert relative to a central axis of a milling cutter (i.e., an axial rake angle) is negative. If the cutting insert is positioned at a negative rake angle, then the cutting insert is subjected to more resistance compared to a positive rake angle. Thus, such a cutting insert is unsuitable for cutting machining with high speed, high feed rate or high depth of cut. To address such a problem, the conventional cutting insert is designed such that the cutting edge is obliquely arranged nonparallel to the upper or lower surface to reduce the negative degree of the rake angle. Nonetheless, the conventional cutting insert still has difficulties in performing an effective cutting in cutting machining with high speed, high feed rate or high depth of cut.